Strong Rail Spin 1/2 Antiferromagnetic Ladder Systems: (Dimethylammonium)(3,5-Dimethylpyridinium)CuX4, X = Cl, Br

The mixed cation salts, (dimethylammonium)(3,5-dimethylpyridinium)CuX4 (X = Cl, Br), henceforth (DMA)-(35DMP)CuX4, are new examples of spin-ladders based on nonbonded halide center dot center dot center dot halide interactions between CuX(4)(2-)anions. In these structures, double rows of the CuX42- anions are sheathed by the 35DMP(+) cations, while the edges are capped by the DMA(+) cations. For the Br salt, the Br center dot center dot center dot Br contacts that define the rungs of the ladder are 4.017 angstrom in length, while those that define the rails are 3.983 angstrom. For the Cl salt, the corresponding lengths are 3.967 and 4.045 angstrom. The susceptibility data for the Br salt exhibits a maximum at similar to 5.5 K, and fitting the data to the spin 1/2 antiferromagnetic ladder model yields 2J(rail)/k = -7.95 K and 2J(rung)/k = -4.07 K. The exchange coupling is much weaker in the Cl salt, no maximum in chi is observed down to 1.8 K, and the corresponding exchange constants are -1.59 and -1.25 K, respectively. An analysis is made of the structural factors involved in the J(rung) pathway.